1. Field of the Invention
The present invention relates to a micro-strip patch antenna for a radiotelephone transceiver.
2. Description of the Related Art
Mobile radio terminals, such as portable radiotelephones, are getting smaller and lighter. In wireless communications, the antenna has a great influence on the performance of the radiotelephone. The antenna is the interface between the radiotelephone and free space. Since most "regular-sized" antennas exhibit close-to-theoretical performance when outside influence is not significant, they typically can easily be designed. Small antennas, however, have low radiation efficiency and a narrow frequency bandwidth. In addition, since a current may be induced in the radiotelephone body by electromagnetic interaction between antenna elements and the radiotelephone body, an electromagnetic wave may be radiated in an unexpected direction.
The types of linear antennas generally used in portable radiotelephones are the .lambda./2 monopole antenna (the length of which is set to half of the wavelength of the electromagnetic wave employed), a .lambda./4 monopole antenna (an improved version of the .lambda./2 monopole antenna), and a .lambda./2 whip antenna. These antennas have a length of 16 or 8 cm when the employed frequency is 900 MHz or 1.9 GHz, respectively, and can be enclosed in the radiotelephone body.
When the 900 MHz band is assigned as the frequency for radiotelephone communication, however, the length of the antenna must be 16 cm so as to receive the electromagnetic wave with the .lambda./2 monopole antenna.
Since the length of the above monopole antennas is relatively long, as depicted in FIG. 1, a radiotelephone using a monopole antenna as described above must use an external antenna 3 which projects outward from the radiotelephone body 1.
In a radiotelephone having such an external antenna, as illustrated in an RF (radio frequency) characteristic curve shown in FIG. 2, it is difficult to attain the maximum gain at the upper and lower limit frequencies actually containing receiving (Rx) and transmitting (Tx) communication signals. Therefore, when the bandwidth is set to be wide (so as to attain the maximum gain), there arises a problem in that noise tends to interfere with the signal wave easily. Further, the monopole type external antenna is an element that severely limits the freedom of the designer in designing the radiotelephone.
A known alternative to the monopole type external antenna is the general micro-strip patch antenna. The general micro-strip patch antenna, although more compact, has several drawbacks, as will now be described.
A general micro-strip patch antenna may use a dielectric ceramic, but requires two dielectric ceramic element parts for transmitting and receiving signals when the transmission and reception bandwidths are different from each other (as is usually the case with portable radiotelephones).
FIG. 3 shows a conventional internal antenna with a transmitting patch 30, a transmitting dielectric ceramic 32, a common ground 34, a receiving dielectric ceramic 36, and a receiving patch 38. In a radiotelephone having separate transmitting and receiving frequency bandwidths, the two dielectric antennas, which respectively perform the transmitting and receiving functions, are bonded to each other with the transmitting and receiving patches 30 and 38 facing outward.
Thus, such an antenna really is two dielectric antennas (one for transmission and one for reception), and it is difficult to reduce the size of a portable radiotelephone using such a general micro-strip antenna.
There are other problems with the general micro-strip antenna. For one thing, the supplying of power from what is typically a sole power supply point to the dielectric antennas is difficult, and it is also difficult to draw a common ground line. Further, the unit price of this type of antenna is high, and they are heavy enough to contribute significantly to the total weight of a radiotelephone. Furthermore, since the power to the antennas is normally supplied through only one channel, there is a disadvantage in that the main circuit of the radiotelephone must be altered because of the use of two antennas.